1. Field of the Invention
The present invention relates to a vibration suppressing method and a vibration suppressing device for a machine tool which is provided with a rotary shaft for rotating a tool or a workpiece to machine the workpiece, and more particularly to a vibration suppressing method and a vibration suppressing device for preventing deterioration of a machining surface and shortening of tool life due to chatter vibration generated during machining of the workpiece.
2. Description of Related Art
In an end mill machining, chatter vibration occurs in accordance with machining conditions such as a cutting amount and a rotation speed of a rotary shaft, which will result in deterioration of a machining surface. The chatter vibration will damage the tool, resulting in shortening of the tool life.
A technique for suppressing chatter vibration is known from Non-patent document 1, and a vibration suppressing method utilizing the technique above is known from Patent document 1. According to the vibration suppressing method, in order to suppress regenerative chatter vibration as self-excited vibration which will cause deterioration of an accuracy in finishing the machining surface, the natural frequency of a chatter vibration-generating system of a tool, a workpiece and the like is obtained by impulsive excitation of the tool or the workpiece, and the obtained natural frequency is increased 60 times and then divided by the number of tool flutes and a predetermined integer to obtain a value which is an optimum rotation speed. Machining of the workpiece is performed using the obtained optimum rotation speed.
Meanwhile, other than the use of the natural frequency of the machine tool, a calculation method using chatter frequency during chatter being occurred is also known from Non-patent document 2 and Patent document 2. According to the disclosure of Patent document 2, an acoustic sensor is arranged in proximity of the tool or the workpiece so as to detect vibrational frequency during the rotation of the tool or the work, and a chatter frequency during machining is obtained based on the detected vibrational frequency. The obtained chatter frequency is increased 60 times and then divided by the number of tool flutes and a predetermined integer to obtain an optimum rotation speed. However, Non-patent document 2 shows that a divisor corresponding to the actual stable range is not equal to an integral multiple of the number of tool flutes. The decimal part of the divisor indicates a phase difference between the surface created by the currently used cutting blade and the surface created by the previous cutting blade, and the rotation speed with no phase difference is obtained as a stable rotation speed in Patent document 2. However, according to Non-patent document 2, an actual optimum rotation speed is slightly higher than the rotation speed with no phase difference. Therefore, the method disclosed in Patent document 2 causes an error in the optimum rotation speed by the phase difference. It is known that a correlation exists between the phase information and the optimum rotation speed, and chatter vibration may be suppressed by obtaining the rotation speed corresponding to the phase information which is to be stable.    Patent document 1: Japanese Laid-open Patent Publication No. 2003-340627    Patent document 2: Japanese Patent Publication No. 2001-517557 for Published Japanese translation of PCT application    Non-patent document 1: CIRP, Vol. 44/1 (1995) Analytical Prediction of Stability Lobes in Milling    Non-patent document 2: Workshop material and papers, 2001-JSME (Japan Society of Mechanical Engineers), Basic knowledge of cutting works and chatter vibration
However, the vibration suppressing method as disclosed in Patent document 1 requires an expensive impulse device, and an excitation using the impulse device requires advanced techniques and great care. Further, in most cases, the natural frequency measured in advance of machining with the impulsive excitation or the like tends to differ from the natural frequency during the actual machining, and therefore it is difficult to accurately obtain the optimum rotation speed based on this method.
Further, since chatter frequency changes in accordance with rotation speed, in order to accurately obtain the optimum rotation speed, it is necessary to obtain the chatter frequency corresponding to the optimum rotation speed. For this reason, according to the vibration suppressing method as disclosed in Patent document 2 in which the measured chatter frequency is used as it is, it is difficult to accurately obtain the optimum rotation speed.
In the meantime, as described above, chatter vibration may be suppressed by obtaining the rotation speed corresponding to the phase information which is to be stable. However, for this reason it is necessary to obtain the chatter frequency corresponding to the intended phase information. Although the phase information and the rotation speed corresponding to a certain chatter frequency can be calculated based on the method as disclosed in Non-patent document 2, it is very difficult to perform inverse transformation using this calculating formula. As a result, from a practical standpoint of view, it is difficult to analytically obtain the chatter frequency corresponding to the intended phase information. Therefore, a calculation should be repeated while sweeping the chatter frequency within a certain definite range, and a certain chatter frequency by which the obtained phase information is sufficiently close to the intended phase information has to be used.
In view of the above disadvantages, the present invention seeks to provide a vibration suppressing method and a vibration suppressing device for a machine tool, which can readily and highly accurately obtain the optimum rotation speed for effectively suppressing chatter vibration, without requiring heavy-loaded repeated computations.
The present invention has been made in an attempt to eliminate the above disadvantages, and illustrative, non-limiting embodiments of the present invention overcome the above disadvantages and other disadvantages not described above.